Process for the direct reduction of metallic oxides

ABSTRACT

A process is described for the direct reduction of metallic oxides, primarily iron ore, to form sponge iron in a direct reduction plant by means of reduction gases containing hydrogen, at least part of the water vapor produced by the reduction process and contained in the exhaust gas being eliminated in the form of condensate. Further, there is described an apparatus for carrying out the process.

BACKGROUND OF THE INVENTION

In the direct reduction of iron ore, for example, in accordance withDT-AS 1 914 400, it is known to use cooling towers to remove heatproduced by the process, and, at the same time to effect evaporation ofthe circulating water. Since the make-up water usually contains acertain salt content, the open water circuit used for this process doesnot only entail losses by evaporation, but also by clarification,because the salt content in the circulating water should not exceed agiven value.

In order to compensate for these losses, a direct reduction plant with acapacity of 400,000 typ (metric tons per year) in dependence on themake-up water quality would need a quantity of make-up water rangingbetween 70 and 100 m³ /h. Any clogging of scrubbers, coolers, andcooling towers, which would lead to a shutdown of the plant and to aproduction loss, can be avoided when the make-up water meets certainquality requirements. In case the untreated water available does notmeet these requirements, the make-up water has to be treated. Untreatedwater of a poor quality necessitates the use of a make-up watertreatment which is rather expensive.

The basic concept of the present invention is to provide make-up waterwithout great difficulties and at minimum cost, the quality of which isadequate for use in a direct reduction plant, even in such locationswhere only untreated water of poor quality is available. Thus,complicated and expensive make-up water treatment is unnecessary. Anyclogging of scrubber and cooler packings is impeded by the supply offully desalted water, and the quantity of salt in the reformer due tothe water drops entrained, is reduced.

This problem is solved according to this invention by directing at leastpart of the purified condensate into the industrial water system,especially into the cooling water system of the direct reduction plant.Moreover, at least part of the cooling water utilized is retained in theform of a closed circuit with indirect heat exchangers as coolers, thelatter being fed with sea water, river water, or air serving as coolingmedia.

In the process according to the present invention, the condensed waterin the top gas scrubber during a direct reduction process is used asindustrial water. This condensed water is nearly free from salt, thuscorresponding to the requirements of the make-up water quality withoutnecessitating any complicated and expensive make-up water treatment forthe elimination of salt.

A direct reduction plant with a capacity of 400,000 typ produces approx.10 m³ /h of fully desalted water. The water is generated by thereduction of iron oxides by means of the hydrogen in the reduction gasesin accordance with the reaction:

    Fe.sub.2 O.sub.3 + 3 H.sub.2 = 2 Fe + 3 H.sub.2 O

although the quantity of water generated per unit of time by thereduction process represents only part of the make-up water required perunit of time for an open cooling water circuit, i.e. a system withdirect heat exchangers, the invention is advantageous even in a plantwith an open cooling water circuit. However, the invention is of specialimportance with combined with a closed cooling water circuit or a partlyopen or partly closed cooling water system. In the case of a directreduction plant with the capacity indicated, the losses (of a closedcooling water circuit) amount to approx. 1-3 m³ /h as regardsclarification, and to approx. 1 m³ /h due to evaporation includinglosses by leakages. These losses can readily be compensated for by thein situ production of 10 m³ /h of fully desalted water. The surplus ofthe fully desalted water produced by the direct reduction process can beused as industrial water or for other purposes, or the cooling watercircuit can be designed partly closed and partly open, an open systembeing preferred so that the quantity of evaporation water willcorrespond more or less to the surplus quantity of condensed water notused in the closed circuit. By means of a control device adjusting themake-up water supply to the open and to the closed system according tothe quantity of condensed water produced the plant can be operatedindependently, without necessitating any external water supply.

When using a closed cooling water system, the process according to thisinvention does not only offer the advantage of the complete directproduction of make-up water, but, moreover, involves the followingadditional advantages:

In direct reduction plants with open water systems, clogging of scrubberand cooler packings occurs at different intervals due to the quality ofthe water. The water quality is of great importance with regard to thecontinuously varying CO₂ content in the circulating water, caused by theCO₂ absorption in the scrubbers and the CO₂ extraction in the coolingtowers.

Due to the high CO₂ content in the top gas and in the cooling gas ofmax. 25% and due to the overpressure of approx. 0.25 atm_(g). in thescrubbers, the water reaches such a CO₂ concentration that there is asurplus of CO₂ in the return water. This charged water reacts with themetallic iron particles, decomposes these particles, and then flows tothe thickener where part of the CO₂ is eliminated.

By irrigating the water in the cooling tower, the remaining CO₂ is ledoff to the atmosphere. Thereby, calcium bicarbonate is reduced tocalcium carbonate, iron bicarbonate and iron sulphate to iron (III)hydroxide insoluble in water in accordance with the following equations:

    Ca(HCO.sub.3) 2→CaCO.sub.3 + CO.sub.2 + H.sub.2 O

    4 fe(HCO.sub.3).sub.2 + O.sub.2 + 2 H.sub.2 O→Fe(OH.sub.3) 3 + 8 CO.sub.2

    4 feSO.sub.4 + O.sub.2 + 1 OH.sub.2 0→4 Fe(OH.sub.3) + 4 H.sub.2 SO.sub.4

the remaining calcium ions precipate in the form of carbonate; the ironions flocculate in iron (III) hydroxide. Having started in thehoneycombs of the cooling tower, this process continues in the coolingtower basin. The flocculated iron and the calcium ions are led to thescrubber packing by the water and are eliminated by the packing servingas a filter. The flocculated iron and the calcium ions are combined inthe resulting slurry.

Thus, with every water circulation a new quantity of metallized ironparticles is decomposed in the scrubbers, oxidized in the cooling towersto iron (III) hydroxide insoluble in water, and eliminated as slurry inthe cooling towers and scrubber packings.

The poorer the water qualtiy, the higher the quantities of impuritieswhich are eliminated. Sulphates and chlorides involve the greatestdisadvantages. By means of the micro-organisms in the water systems, thesulphates are reduced to H₂ S. H₂ S and iron form FeS, whichprecipitates in form of a slurry.

When replacing the cooling towers by indirect heat exchangers, the CO₂variations in the system water are eliminated, thus, impeding thereduction of iron from its metallic form to Fe (III) hydroxide, whichhas a positive effect on the quality of the circulating water.

The problem is particularly acute in large plants where it is necessaryto cope with higher gas quantities and higher pressure losses in the gassystems. Conventional rotary compressors have to be replaced by turbocompressors, which, in the case of open water systems, involves a greatrisk of damage to the apparatus employed.

Water particles, carried over from the scrubbers, which certaindissolved salts in the case of open water circuits evaporate due to thecompression heating of the gas. The evaporation residues (ferrousparticles and salts) deposit at the compressor (particularly on theimpeller and casing), to which they become strongly bound. Thusunbalances and defects of the rotor as well as stoppages occur. Contraryto the open water-cooling system, the closed water circuit is fullydesalted. The iron particles, absent the presence of salt which acts asa binding agent cannot result in extensive deposits. Therefore a closedwater circuit enables the use of turbocompressors.

In order to exactly control the vapor content of the process gas, aprocess gas aftercooler has to be connected when rotary compressors areemployed. In the process gas aftercooler, the heat produced bycompression is destroyed and the gas is cooled down to 70-75° C.Afterwards this gas is heated up to approx. 400° C. in the recuperators.

When using turbocompressors the water vapor content can be controlledwithout the necessity of a process gas aftercooler in the process gaszone of the top gas scrubber.

The gas heated to approx. more than 200° C. in the turbo-compressor canbe led directly to the recuperator for further heating. Thus lowerinvestment costs and a decrease of the specific heat consumption areachieved.

By using a closed water circuit for the discharge of the excessiveprocess heat in direct reduction plants, make-up water is only necessarybefore the start-up of the plant for the first filling of the closedcircuit system. On the beginning of production excessive quantities offully desalted water are produced in the reduction furnace, part ofwhich is fed to the circuit to cover the evaporation and leakage losses.

Moreover the conventional clarifying of the thickened circuit waterbecomes unnecessary when using a closed water circuit. The plant doesnot furnish sewage because the remaining quantity of the produced whichis not needed as make-up water can be fed to another installation, e.g.to a steel plant as make-up water.

The closed water circuit also shows positive effects on process gasproduction in a catalytic gas reformer when the latter is an integratedpart of the total reduction process.

More specifically, the catalyst bed of the gas reformer is inactivatedby the presence of sulphate, chloride and other salts contained in thewater particles, i.e. the efficiency of the gas reformer is decreased.The fully desalted water provided by the closed circuit of the presentinvention however, contains no salts which are harmful to the catalyst.Thus, aging of the catalysts due to harmful components of the watersystem is substantially eliminated.

The invention is more particularly described by reference to theaccompanying drawings, wherein:

FIG. 1 shows a direct reduction plant with a closed water coolingsystem; and

FIG. 2 shows a direct reduction plant with an open water cooling system.

In the direct reduction plant shown on FIG. 1, iron ore 2 charged fromabove is reduced to sponge iron 4 in a shaft furnace 1 by means ofreduction gases fed through a gas inlet 3. The sponge iron is extractedthrough an opening in the bottom of the shaft furnace. In the bottompart of the shaft furnace a cooling water circuit 5 is provided forcooling the sponge iron before leaving the furnace. A cooling gasscrubber is included in the cooling circuit. In a gas reformer 7, thereduction gas comprises hydrocarbons and oxidizing agents which are fedto a mixing facility 8. The main oxidizing agent is the top gas orexhaust gas respectively, emerging through an upper gas outlet 9 of theshaft furnace 1, after it is purged and cooled in a top gas scrubber 10.

The water vapor contained in the exhaust gas of the shaft furnace isdischarged as condensate in the top gas scrubber 10 and is led partlyvia a coarse grain screen 12 and partly directly via a discharge pipe 13to a mixing facility 14 from where it is led to a thickener 16 togetherwith a flocculant from a flocculant station 15. Moreover the watercircuit of the cooling gas scrubber 6 is connected to the coarse grainscreen 12. For reasons of simplification that is not shown on thedrawing. The thickener is connected to the mixing facility 14 by areturn line 17 and besides a discharge pipe 18 for the contaminatedwater an overlow 19 is provided for the industrial water. This is fed toindirect heat exchangers 20 and 21. The number of heat exchangers is notlimited but depends on the requirements of the process. From the heatexchangers, the cooled water is directed to a collecting tank 22, fromwhere it is again led to the top gas scrubber or to other areas notshown. In this case sea water is used as cooling agent for the heatexchangers 20 and 21. It is led to the heat exchanger via a feed pipe 23and discharged via a discharge pipe 24. Of course, river water or aircan also be used as the cooling agent.

In direct reduction plants of conventional size, the quantity ofcondensed water separated from the exhaust gas is more extensive thanthe losses occurring in the closed water circuit. Accordingly, cooledand fully desalted water for other purposes e.g. for the water supply ofa nearby steel plant can be taken from water tank 22.

FIG. 2 shows a direct reduction plant with an open water cooling system.As far as this plant is identical with the plant shown on FIG. 1 thesame reference symbols are employed. In contrast to the plant of FIG. 1,a cooling tower 25 is provided instead of the direct heat exchangers.The cooling tower represents a larger number of cooling towers.

The cooling tower is provided with a collecting tank 26 receiving purgedwater from the overflow 19 of the thickener 16 which is then fed intothe cooling tower 25 from above via a pipe 27. The cooled water isdirected to a collecting tank 28 and from there is again led to the topgas scrubber if necessary after the addition of water from thecollecting tank 26.

It is within the scope of the present invention to employ a combinationof a closed water cooling system according to FIG. 1 and an open watercooling system according to FIG. 2, the open water cooling circuit beingoperated in a way that the evaporation in this system basicallycorresponds to the production of condensed water less any lossesresulting from leakage and sludge. In such a plant, part 31 containingthe cooling tower indicated on FIG. 2 by a dash-dot line, is connectedin parallel at the points on FIG. 1 marked by crosses 29 and 30. Therequired distribution between the open and the closed cooling watersystems is effected by valves and a control device not shown in thefigures.

What is claimed is:
 1. In a process for the direct reduction of metaloxide to iron sponge by means of reducing gases containing hydrogen gasin a reduction apparatus in which at least a portion of the water vaporformed in the reduction process and contained in the exhaust gas isseparated as condensate by means of cooling water, the improvementcomprising combining the condensate with the residual cooling water andreusing said condensate for cooling purposes, the water being cooledpartially in an open and partially in a closed system, the two systemsbeing arranged in parallel, the condensate produced being sufficient tocompensate for the water evaporated in the open system and removed inthe sludge produced by said system as well as for other normal leakagelosses of cooling water.